High-speed amplifier circuits are used for amplifying signals in a wide variety of employments, wherein a variable gain amplifier (VGA) varies its gain depending on a control signal. To obtain a stable process and temperature independent gain an accurate control signal must be provided, wherein the control signal should take various varying operating conditions into account, for example, temperature or aging processes.
High-speed VGAs can not be designed based on an adjustable ratio of a feedback network, for example, as known from conventional resistive feedback paths, which are operated in negative feedback around an operational amplifier. The bandwidth in such a closed-loop system is determined by the bandwidth of the operational amplifier (OpAmp) that can be achieved with sufficient phase margin at minimum gain setting. Usually the serviceable bandwidth of an OpAmp degrades proportionally to the effective gain of the system. Any pole present in the feedback network reduces the obtainable phase margin thus limiting the available bandwidth of the closed-loop. In cases requiring high adjustability extra components, for example, switches, must be added to the feedback network. These components add to parasitic capacitances present in the feedback network, such that a gain dependent feedback pole may arise. Hence high-speed variable gain amplifiers are commonly designed as open-loop systems.
To obtain a stable process and a temperature independent gain, tuning circuits can be added to an open-loop amplifier arrangement as means for providing a constant gain. Often a two-point calibration of the gain versus gain-control signal relation is implemented in which both the maximum and minimum gains are controlled by extra circuitry. Based on these extreme settings intermediate gain values can be produced.
In a conventional example, for tuning circuitry, copies of the main amplifier, so-called replica circuits, were used for generating control signals. That is the amplifier circuit comprises an open-loop main amplifier for amplifying the input signal as desired. Control signals for adjusting the amplification of the main amplifier are generated in parallel operated replica circuits comprising essentially identical elements, in particular, an identical amplifier. A first replica circuit is employed for producing a first control signal for minimum gain and a second replica circuit is employed for producing a second control value for adjusting maximum gain value. The replica circuits are operated in a closed loop, i.e., the circuits comprise a feedback loop. A reference input signal is provided to each replica circuit and each replica circuit comprises a resistive divider to adjust control values for a minimum and maximum gain. The control signals produced in the replica circuits are coupled to the main amplifier.
As an ever-challenging problem the power consumption of the amplifier circuit must be minimized. Accordingly power consumption of peripheral devices such as the tuning circuits should have a minimum impact on the power consumption of the total variable gain system.
Hence there is a need for a new circuit for controlling the amplification of an input signal.